Signaling System Number 7 - SS7 defines the procedures for the set up, ongoing management, and clearing of a call between telephone users. It performs these functions by exchanging telephone control messages between the SS7 components that support the end users' connection.

The SS7 Signaling data link is a full-duplex, digital transmission channel operating at 64 kbit/s. Optionally, an analog link can be used with either 4 or 3 kHz spacing. The SS7 link operates on both terrestrial and satellite links. The actual digital signals on the link are derived from pulse code modulation (PCM) multiplexing equipment or from equipment that employs a frame structure. The link must be dedicated to SS7.

SS7 was first published in 1980 as a predecessor to SS6 (Signaling System No 6) with extensive improvements published in 1984 and again in 1988. Today SS7 and variations are implemented throughout the world. Indeed, SS7 has found its way into other communications architectures, such as personal communications services (PCS) and global systems for mobile communications (GSM).

Here is a brief and partial summary of some of the SS7 functions: -Setup and clear down a telephone -Provide the called party's number (caller id) -Indication of national, international, or other subscriber -Indication that called party has cleared - Nature of circuit (satellite/terrestrial) -Indication that called party cleared, then went off-hook again -Use of echo-suppression -Notification to reset a faulty circuit -Language of assistance operators -etc.